For small volumes of gas, according to kinetic theory of gases all parts of the gas are at the same temperature. But for huge volumes of gas like atmosphere, the assumption of a uniform temperature throughout the gas is not valid. Apart from the surface of earth, variation in temperature also occurs at different heights in atmosphere. The decrease in temperature with height called the "Atmospheric lapse rate" is similar at various locations it is found that average global lapse rate is −6.7∘ C/km. The linear decrease in temp with height occurs in the lower part of atmosphere called troposphere. Absorption of sunlight at earth's surface warms the troposphere from below. So convection currents are continually produced in the air, and causes its mixing.As a parcel of air rises, its pressure drops and it expands. The parcel does work on its surroundings thus its internal energy and temperature drops.Assume that the vartical mixing of air is so rapid as to be adiabatic and the quantity TP(1−γ)γ has a uniform value throughout the layer of troposphere.(Given: M is molecular weight of air, R being the universal gas constant, P and T are pressure and temperature respectively at a point under consideration at a height y)If the mechanical equilibrium of atmosphere requires that the pressure decreases with altitude according to dpdy=−ρg consider g to be constant, the lapse rate is given by: